[RFC] Doxygen neovim.org theme

A picture is worth a thousand words:
[![devdocneovimtheme](https://cloud.githubusercontent.com/assets/393493/3210941/9ec49c30-eef1-11e3-8a61-96954a802e12.png)](http://neovim.menkar.uberspace.de/devdoc/neovim_theme/fs_8c.html)

http://neovim.menkar.uberspace.de/devdoc/neovim_theme/

ref https://github.com/neovim/neovim.github.io/issues/48
Specific suggestion: the doxygen theme/support files should go in `/contrib/doxygen/` (no need for a "theme" subdirectory, after the above changes)